1. Field of the Invention
The present invention relates to an optical recording medium and method, which record information by irradiating a laser beam to a recording layer so as to form a recording mark, and in particular, to a technology of forming a plurality of recording marks having different state so as to record a data at a multi-level.
2. Description of the Related Art
In a conventional optical recording medium, a method has been employed such that a data is recorded by changing a length of read signal (i.e., a length of reflection signal modulation section). On the contrary, the following method has been frequently studied, more specifically, a plurality of data is recorded to each signal having the same length by changing a depth of the read signal (i.e., a modulation degree of reflection signal) at multiple stages.
According to the above optical recording method, as compared with the case where a binary data is merely recorded by the presence of pit, a plurality of data is recorded in a depth direction, so that a signal quantity allocated to a constant length can be increased; therefore, it is possible to improve a track recording density. In general, it is well known to change a power of laser beam at multiple stages, as the method of a depth of read signal at multiple stages. Recently, holograph has been used as the recording medium, and a recording medium having a multiple recording layer has been proposed.
In this case, it is called as multi-level recording to record each data so that a modulation degree of reflection signal is variable at multiple stages.
In the above multi-level recording, a laser beam is irradiated at multiple-stage power in recording; for this reason, in particular, when the power of laser beam gradually becomes large, a problem has arisen such that a signal quality is worsened in reading. Although the above cause has not been proved, the present inventors presume the cause as follows; more specifically, the laser power increases, and thereby, a recording mark area increases.
For example, in order to achieve a high density of the recording information of recording medium, in the case where multi-level recording is carried out in a manner of making small a recording mark, and changing a laser power at multiple stages, a signal quality has remarkably worsened in the recording mark recorded by large laser power.
Namely, if the multi-level recording method is employed using power change, a distance (interval) between the recording marks is made wide, and data detection must be securely made to some degree even if the signal quality has worsened.
Moreover, the conventional concept of achieving the multi-level recording by changing the laser power stepwise is based on the following premise that the recording mark length is at least larger than a radius of focused beam (beam waist) in recording. In general, a diameter of the focused beam is expressed as Kλ/NA (K: constant, λ: laser wavelength, NA: numerical aperture of lens. For example, in a pickup used for a CD, the laser wavelength λ is 780 nm, the numerical aperture NA is 0.50, and the diameter of focused beam is about 0.8 μm. Thus, when the recording mark length is made small up to the vicinity of about 0.8 μm, the above problem of signal deterioration has remarkably arisen. In fact, it is very difficult to achieve five stages or more multi-level recording by the method of changing a laser power.
On the contrary, although is unknown when this application has been filed, an optical recording method has been disclosed in Japanese Patent application No. 2000-187568 filed by the same applicant as this application. More specifically, according to the above optical recording method, in place of the power of laser beam, an irradiation time to a predetermined virtual recording cell area is changed at multiple stages. By doing so, a recording mark (low optical reflectance area) having different size is formed and brings different optical reflectance to the entire virtual recording cell; therefore, at least five stages multi-level recording is possible.
The above optical recording method is a concept quite different from the conventional laser beam irradiation, and probably includes an extremely short irradiation time. Therefore, the case is sufficiently considered such that a recording mark smaller than a beam spot diameter is formed. In other words, multi-stage (five stages or more) and high density multi-level recording can be realized by effectively using an area in which the recording mark has been conventionally considered as being incomplete.